Selenium supply regulates thyroid function, thyroid hormone synthesis and metabolism by altering the expression of the selenoenzymes Type I 5'-deiodinase and glutathione peroxidase.

Selenium supply appears to be insufficient in several regions of Europe as indicated by analysis of plasma and tissue selenium content and measurements of selenium dependent glutathione peroxidases (GPx). Selenium deficiency alters both thyroid hormone synthesis and tissue specific activation by 5'deiodinase isoenzymes. Human and rat Type I 5'deiodinase have been identified as selenoprotein containing selenocysteine in its active site. Regulation of selenium dependent expression of Type I 5'deiodinase occurs at the transcriptional and posttranscriptional level and a cellular hierarchy of selenite incorporation was found for Type I 5'deiodinase over GPx in LLC-PK1 kidney cells. The role of other newly discovered selenoproteins for thyroid hormone synthesis, activation, and action has to be investigated.